Retorting system

ABSTRACT

Hot porous pellets are cycled to a retort zone to mix with and retort preheated crushed oil shale, thereby producing gas and oil products, a combustible deposition on the pellets, and a mixture of pellets and spent shale. The main source of heat for retorting is derived from controlled burning of the deposition on the pellets in a pellet deposition burning zone. The hot flue gas thus generated is passed to a secondary preretort oil shale heating zone to additionally preheat already preheated oil shale. Thereafter, the hot flue gas is passed to a gas elutriation system where the mixture of spent shale and pellets is processed to separate and recover the pellets for return to the retorting process. From the gas elutriation system, the hot flue gas is used to lift the recovered pellets to the pellet deposition burning zone. Thereafter, the hot flue gas is passed to an initial preretort oil shale heating zone to preheat raw crushed oil shale before it is fed to the secondary preretort heating zone. The hot flue gas from the burning zone usually contains carbon monoxide and may, at any desirable point in the system, be passed through a carbon monoxide shift reaction zone.

United States Patent [191 Abdul-Rahman 1 Apr. 9, 1974 RETORTING SYSTEM Yahia A. K. Abdul-Rahman, Plano, Tex.

[73] Assignee: Atlantic Richfield Company, New

York, N.Y.

221 Filed: Dec. 26, 1972 [2]] Appl. No.: 318,209

[75] Inventor:

Primary Examiner-Curtis R. Davis [57] ABSTRACT Hot porous pellets are cycled to a retort zone to mix with and retort preheated crushed oil shale, thereby producing gas and oil products, a combustible deposition on the pellets, and a mixture of pellets and spent shale. The main source of heat for retorting is derived from controlled burning of the deposition on the pellets in a pellet deposition burning zone. The hot flue gas thus generated is passed to a secondary preretort oil shale heating zone to additionally preheat already preheated oil shale. Thereafter, the hot flue gas is passed to a gas elutriation system where the mixture of spent shale and pellets is processed to separate and recover the pellets for return to the retorting process. From the gas elutriation system, the hot flue gas is used to lift the recovered pellets to the pellet deposition burning zone. Thereafter, the hot flue gas is passed to an initial preretort oil shale heating zone to preheat raw crushed oil shale before it is fed to the secondary preretort heating zone. The hot flue gas from the burning zone usually contains carbon monoxide and may, at any desirable point in the system, be passed through a carbon monoxide shift reaction zone.

3 Claims, 1 Drawing Figure 1 RETORTING SYSTEM BACKGROUND OF THE INVENTION This invention relates to a retorting system wherein hot porous pellets are used to retort preheated crushed raw oil shale and are separated and recovered from spent shale particles, are lifted to a burning zone, reheated and recycled through the process, and wherein the hot flue gas from the burning zone is sequentially passed through several stages of the process.

As a preliminary stage inthe production of petroleum oils and gases, the solid carbonaceous organic solid matter or kerogen in oil shale is pyrolyzed or retorted. Retorting denotes thermal conversion of kerogen or organic matter to oil vapors and gas, thereby leaving solid particulate spent shale and includes separation of the oil vapors and gas from the spent shale. The spent shale contains residual carbonaceous organic matter and matrix mineral matter.v

When the kerogen is retorted, a normally gaseous fraction, a normally liquefiable vaporous fraction, and a combustible organic residue are formed. The product distribution between gas, liquid, and residue is important and relates to the distribution of the various boiling point fractions in the liquid product. Copending applications Ser. No. 284,288, filed Aug. 28, I972; Ser. No. 285,732, filed Sept. 1, 1972; Ser. No. 304,074, filed Nov. 6, 1972; and Ser. No. 308,136, filed Nov. 20, 1972, which are owned by a common assignee and are incorporated herein, provide processes for retorting oil shale using special pellets in a way which causes a combustible organic carbon deposition to be formed on the pellets during retorting of oil shale and improves the recovery of useful components and liquid product distribution. Some or all of this combustible deposition is burned in a pellet deposition burning zone to heat and reheat the pellets. In such processes, the spent shale is separated from the pellets prior to burning and the pellets are recycled through the process. Preferably, separation will involve hot gas elutriation in one or more stages.

In retorting processes, especially processes of the aforementioned nature, fuel requirements and the amount of fuel used for heating purposes should be kept as low as possible. Moreover, due to restrictions on gas emissions and the costs and difficulties of cleaning gas emission streams, it is highly desired that the number of gasemission points be kept as low as is practical.

SUMMARY OF THE INVENTION The retort system of this invention provides a method of retorting using hot porous pellets and sequential passage of hot flue gas through a secondary crushed oil shale preheating stage, a spent shale separating zone using gas elutriation, a pneumatic pellet lifting system, and an initial oil shale preheating stage all in a way which conserves gas heating values, reduces fuel requirements, and minimizes gas emissions.

Mined oil shale which contains solid carbonaceous organic matter and other mineral matter and which has been crushed is preheated and retorted in a retort zone with hot pellets at a temperature and in an amount sufficient to provide at least 50 percent of the sensible heat required to retort the oil shale. Retorting oil shape produces gas and oil products which are recovered, and particulate mixture of pellets and spent shale. The requentially passed in aforementioned fashion.

BRIEF DESCRIPTION OF THE DRAWING The drawing is a partly schematical, partly diagrammatical, flow illustration of a system for carrying out a preferred sequence of the process of this invention.

DETAILED DESCRIPTION OF THE INVENTION A process for retorting crushed oil shale containing a carbonaceous organic matter, commonly called kerogen, and other mineral matter with hot pellets which are later separated and recovered from the spent retorted shale is described having reference to the drawmg.

Pellets bearing a combustible deposition which was previously absorbed or deposited on the pelets in an earilier stage of the retorting system are passed through pellet return line 11 whichacts as an inlet line to pneumatic pellet lift system 12 where the pellets are lifted by way of lift line 13 to pellet deposition burning zone 14. Preferably, the pellets will be lifted to an elevation which allows gravity feed of the pellets to a retort zone. The pneumatic pellet conveying system operates in the conventional manner to lift and convey the pellets except that the lift gas which enters the lift system via line 15 and moves through lift system 12 at a velocity sufficient to elutriate or lift the pellets, for example, 25 to feet per second, is at least in part comprised of a hot flue gas which originated in pellet deposition burning zone 14 as hereinafter shown.

As shown, pellet burning deposition burning zone 14 is comprised of surge hopper 16 for collecting the lifted pellets and leveling out fluctuations and from which the pellets fall into burning units 17, The hot lift gas passes overhead of the surge hopper through exit line 18 where the hot lift gas is passed to oil shale first preretort heating zone 19 which may be comprised of one or more heating units. Raw or fresh oil shale which has been mined and pulverized, crushed or ground for the most part to a predetermined maximum size for handling in a retort system by any suitable particle diminution process and which is at a temperature lower than the hot lift gas is fed directly from a crusher or from a hopper or accumulator by way of shale inlet line 20 into first preretort heating zone 19. In the zone, the hot lift gas lifts, intimately contacts, and heats the cooler crushed oil shale to a temperature not to exceed 600 F in order to avoid retorting or loss of oil values in the raw shale. The hot lift gas and preheated oil shale pass overhead through exit 21 to collection chamber 22 where the oil shale settles from the lift gas and is gravity or mechanically fed through line 23 to second preretort heating zone 24. The separated hot lift gas passes overhead of collection chamber 22 through exit line 25 where the gas may be passed to additional upstream preretort oil shale heating units.

Crushing of the raw mined shale expedites more uniform contact and heat transfer between the shale feedstock and hot lift gas andin a retort zone between the,

shale and pellets. In normal practice, the degree of crushing is simply dictated by an economic balance between the cost of crushing and the advantages to be gained by crushing when retorting the kerogen from the shale. Generally the shale feedstock is-crushed to about 7% inch and no particular care is taken to produce or restrict production of finer materials.

In burning units 17, the combustible deposition on the pellets is burned to provide at least 50 percent or more of the heat required to reheat the pellets to the temperature required to effect retorting of the shale. The combustible deposition is burned in a manner similar to the way that catalytic cracking catalysts particles are regenerated. and which is controlled to avoid excessive-heating of the pellets which would excessively reduce the effective surface area of the pellets to less than 10 square meters per gram. A progressive bed burner is preferred. A combustion supporting gas, for example air, a mixture of air and fuel gas generated in the process, and flue gas with the desired amount of free oxygen, is compressed or blown into the pellet deposition burning zone at a temperature at which the deposition on the pellets is ignited by way of combustion gas inlet 26. Steam may also be used to control burning provided that the steam does not excessively reduce the surface area of the pellets. The combustion supporting gas may be preheated in heaters 27 by burning some of the gases produced in the process to reheat the pellets to the minimum ignition temperature. The quantity of combustion supporting gas, e.g., about 2 to 3 pounds and higher depending on compression pressure of oxygen per pound of carbon deposit, affects the total amount of deposition burned andthe heat generated by such burning and in turn the temperature of the pellets. The bulk density of the pellets is about 40 to 50 pounds per cubic foot and the specific heat of the pellets varies between about 0.2 and 0.3 British Thermal Units per pound per degree Farenheit. The gross heating value of the carbon-containing deposition is estimated to be about l5,000 to 18,000btu per pound. The amounts of carbon dioxide and carbon monoxide produced in the flue gases created by burning the pellet deposition indicate the amount of combustion supporting gas required or used and the amount of carbon-containing deposit riot burned. Generally, it is desirable to attempt to free the pellets of all of the combustible deposition. Other factors taken into consideration during burning of the pellet deposition are the pellet porosity, density, and size, the burner chamber size and pellet bed size, residence burning time, the desired temperature for the pellets, heat losses and inputs, the pellet and shale feed rates to the retort zone and the like. The residence burning time will usually be rather long and up to about 30 to 40 minutes. Combustion of the deposition should be controlled in a manner which does not heat the pellets to above 1,500 F. Of course, additional fuel material or gases may be used to supplement burning of the pellet deposition if this is necessary.

The hot flue gases generated in the pellet deposition burning zone are passed through lines 28 to second preretort heating zone 24, which may be comprised of one or more heating units. Generally, as previously indicated, the hot flue gases will contain carbon monoxide which can be used in a conventional manner in a carbon monoxide shift reactor wherein the carbon monoxide is changed to carbon dioxide thereby exothermically producing additional heat. Therefore,

whenever additional heat is required or if at some point an excess of heat is present, the hot flue gas may be passed through an optional carbon monoxide shift reaction zone. lf an excess of heat is present, the heat can be used for boiler operation or the like. The carbon monoxide shift reaction zone may, as indicated, he located at any appropriate point along'the path of the flue gas from the pellet deposition burning zone to the last stage of process wherein the flue gas is used. As shown, the hot flue gas containing carbon monoxide is first passed through shift reaction zone 29 before being passed to secondary preretort heating zone 24.

If the hot flue gas contained an excess of oxygen, supplemental fuel could be added to consume the oxygen or the oxygen converted by other means.

The hot flue gases via line 30 entering secondary preretort heating zone 24 are hotter than the preheated crushed oil shale entering the zone from the first preretort heating zone. The hot flue gas lifts, intimately contacts, and heats the oil shale to a higher temperature which temperature does not exceed about 600 F. The hot flue gas and heated oil shale pass overhead through exit 31 to collection chamber 32 where the oil shale settles from the hot flue gas and is gravity or mechanically fed through retort inlet line 33 to retort zone 34. The separated hot flue gas passes overhead of collection chamber 32 through exit line 35 where the gas may be passed to additional preretort oil shale heating units (not shown) or passed to a separation zone as hereinafter described.

In the pellet deposition burning zone, there is produced a continuous stream of hot pellets having a temperature above 1,000 F and not exceeding l,500 F. As used herein, the term pellets refers to subdivided or particulate heat-carrying bodies having, when they leave the combustion zone, a minimum relatively high effective surface area of 10 square meters per gram and higher, a size range between about 0.06 inch and 0.3 inch, including a narrower range therewithin, and a shape and density or particle weight such that for a particular size the elutriation velocity of -a pellet of that size is significantly, that is, measurably and usefully, higher than the elutriation velocity of most of the particles of spent shale of that size. Preferably, the average surface of the pellets will be between 10 and square meters per gram. The surface area is the average effective of the pellets as they enter the pyrolysis zone. The surface area may be determined by the conventional nitrogen absorption method. The pellets may be cylindrical shape, approximately oval or spherical shape, or purely spherical shape. The much preferred pellets have a sphericity factor of at least 0.9 which shape gives the highest particle weight and is particularly useful in separating the pellets by elutriation and screening from spent shale produced in the retort zone. The sphericity factor is the external or geometric surface area of asphere having the same volume as the pellet divided by the external surface area of the pellet.

The pellets are made up of materials such as alumina or silica alumina, which are not consumed in the process and which are subdivided or particulate matter having significantly high internal surface area. The pellets are sufficiently wear or breakage resistant and heat resistant to maintain enough of their physical characteristics under the conditions employed in the process to satisfy the requirements of the p'rocess,'to affect retorting of the oil shale, and to permit controlled burning of a carbon-containing deposition formed on the pellets during the process. More specifically, the pellets do not disintegrate or decompose, melt or fuse, or undergo excessive surface area reduction at the temperatures encountered during such burning and the thermal stresses inherent in the process.

The pellets do, of course, undergo gradual wear or size reduction. Fresh pellets will usually be between 0.15 and 0.3 inch with a range between 0.2 and 0.3 being initially sought. After wear or attrition, the size range will increase, and great effort is made to retain and recover all pellets above about 0.06 inch or plus 14 US. Sieve Series Screen size. Finer grain pellet-like particles which were once part of the pellets may be present, but no special effort is made to retain these finer grains and a large percentage of substantially smaller finer grained pellets is undesirable.

The stream of hot pellets is fed by gravity or other mechanical means to retort zone 34 by way of pellet inlet pipe 36. The pellets and shale feedstock could be fed to the retort zone by way of a common retort zone inlet. The hot pellets are at a temperature ranging between l,000 F and 1,500 F, which is about 100 F to 500 F higher than the designed retort temperature within the retort zone. The most favorable practical temperature range depends on other process variables. The quantity of pellet heat carriers is controlled so that the pellet-to-shale feedstock ratio on a weight basis is between 1 and 3. This ratio is such that the sensible heat in the pellets is sufficient to provide at least 50 percent of the heat required to heat the shale feedstock from its retort zone feed temperature to the designed retort temperature. The feedstock feed temperature is the temperature of the oil shale after preheating, that is the temperature of the shale upon entry into the retort. The average retort temperature ranges between about 850 F and l,200 F, depending on the nature of the shale feedstock, the pellet-to-shale ratio, the product distribution desired, heat losses, and the like.

The retort zone is any sort of retort which causes intimate contact or mixing of the crushed oil shale and pellets. The preferred retort is any sort of horizontal or inclined retorting drum that causes the oil shale and pellets to undergo a tumbling action. This sort of retort is herein referred to as a rotating retort zone. This type of retort zone is quite flexible over a wide range of conditions and has the advantages of causing rapid solidto-solid heat exchange between the pellets and shale feedstock thereby flashing and pyrolyzing the oil and gas vapors from the shale in a way which allows the vapors to separate from the solids without passing up through a long bed of solids and which minimizes dilution of the product vapors by extraneous undesirable retorting gases, of allowing for a high shale throughput rate at high yields for a given retort volume, of providing for greater control over residence time, of aiding in preventing overcoking and agglomeration of the pellets and shale, of facilitating formation of a more uniform controlled amount of combustible carbon-containing deposition on the surface area of the pellets, and of causing flow of the pellets and shale through the retort zone in a manner which aids in eventual separation of the pellets from the spent shale.

The retorting process is carried out in concurrent or parallel flow fashion with the hot pellets and the raw shale feedstock being fed into the same end of the retort. The retort zone may be maintained under any pressure which does not hamper efficient operation of the retort, interfere with production of valuable retort vapors, or cause excessive deposition of residue on the pellets. Generally, pressurization of the pyrolysis or retort zone causes considerable difficulties, especially if a rotating retort zone is used. The pressure employed is, therefore, generally the autogenous pressure.

in theretort zone, the hotter pellets and cooler crushed shale feedstock are admixed and intimately contacted almost immediately upon being charged into the retort zone. The shale particles are rapidly heated by sensible heat transfer from the pellets to the shale. Any water in the shale is distilled and the kerogen or carbonaceous matter in the shale is decomposed, distilled, and cracked into gaseous and condensable oil fractions, thereby forming a valuable vaporous effluent including gas, oil vapors, and superheated steam. Pyrolysis and vaporization of the carbonaceous matter in the oil shale leaves a particulate spent shale in the form of the spent mineral matrix matter of the oil shale and relatively small amount of unvaporized or coked organic carbon-containing material.

As the aforementioned vaporous effluents are formed, a combustible carbon-containing deposition or residue will be formed or deposited on the pellets if the effective surface area of the pellets has not already been covered with all of the deposition that it can sustain. The total amount of deposition formed or deposited on the pellets upon one passage through the process is sufficient upon combustion to provide at least 50 percent of the heat required to reheat the pellets. The amount of combustible deposition deposited on the pellets during the retorting stage is on an average less than 1.5 percent by weight of the pellets, and the preferred range is between 0.8 and 1.5 percent. The pellet surface area, size, and amount coacts with other process conditions to accomplish the desiredamount of combustible deposition and product distribution. If the surface area of the pellets is less than 10 square meters per gram, either too little total deposition will be formed or the burning of the deposition will not be sufficient to provide a major portion of the heat required to heat the pellets to the desired temperature and to carry out the retorting phase of this process. This would necessitate the use of supplementary fuels which is undesirable.

The mixture of pellets and shale moves through the retort zone toward retort exit 37, and the gaseous and vaporous effluents containing the desired hydrocarbon values separate from the mixture. The residence time for the pellets required to effect retorting and deposition of the pellet deposition is on the order of about 3 to about 20 minutes with residence times of less than 12 minutes for the pellets being preferred. The shale residence time depends on its flow or movement characteristics and since the shale is not uniform in size and shape, the shale residence time varies.

The mixture of pellets and spent shale exits from retort zone 34 at a temperature between 800 F and 1 ,050 F by way of retort exit 37 into recovery chamber 38 where the gaseous and vaporous products, resulting I and handled with other spent shale. Hot dust or fines separation may be accomplished by hot gas cyclones, quenching and washing, agglomeration with sludge or a separately condensed heavy product fraction, centrifuging, filtration, or the like.

The retort zone particulate mixture of pellets and spent shale are passed through separation zone 40 for separation of the spent shale from the pellets and for recovery and return of the pellets to the pellet deposition burning zone. The separation zone may be any sort of exiting and separation system accomplishing at least 75 percent by weight separation of the total spent shale from the pellets and at least 95 percent by weight separation of the spent shale that is smaller than the pellets, provided that, at least one stage of the separation zone involves gas elutriation of at least part of the spent shale. The separation zone may be comprised of any number and types of units of equipment. As shown, the mixture of pellets and spent shale first passes through optional revolving screen or trommel 41 extending into product recovery chamber 38 and which has openings or apertures sized to pass the pellets and spent shale of the same or smaller size than the pellets and screen out any spent shale and agglomerates larger than the pellets. Most of the spent shale and the pellets flow through the openings in trommel 41 and drop to the bottom of recovery chamber 38 to exit via exit line 42. Any spent shale, mineral matter, and agglomerates too large to pass through the openings in the trommel, pass outward through exit 43 to spent shale disposal. This prescreening or initial separation of spent shale larger than the pellets is optional and may also be delayed until a later or final stage of the separation system. The rotating retort makes initial screening easier.

The spent shale and pellets in the bottom of recovery chamber 38 are discharged via exit line 42 at a temperature between about 750F and l,O50 F where these particulate solids are passed or conducted by gravity or other means of conveyance to a subsequent phase of the separation and recovery zone involving at least one stage wherein the mixture of pellets and spent shale is subjected to elutriation gas, preferably preheated or already hot, to elutriate and separate a part of the spent shale from the pellets. The elutriation gas is, at least in part, comprised of flue gas which originated in pellet deposition burning zone 14 and was used in secondary preretort oil shale heating zone 24. As shown, the flue gas in line 35 is first passed through optional heater 44, wherein, if desired, the flue gas can be reheated. Heater 44 could be a carbon monoxide shift reaction zone as previously discussed or could use other means of direct or indirect heating including additional fuel burning. It should be noted. however, that the flue gas will already be hot and the need for additional fuel is, therefore, reduced. The hot flue gas then passes through line 45 to elutriation zone 46. The elutriation zone may be in one or more stages conducted in batchwise fashion or in a combination of batch and separate units, or in two or separate units or stages, as disclosed in Copending Application Ser. No. 318,190, filed Dec. 26, 1972 by the same inventor, entitled Separating Retorted Shale from Recycled Heat-Carrying Pellets, and owned by a common assignee. Theinternal operation and design of gas elutriators is well known and depends on the properties of the particles and such factors as free board height, bed height and weight, gas type and velocity, column diameter and cross-sectional area, and

transport disengaging height. In this process, it is highly desirable that the elutriating gas be maintained at a temperature above 750 F, and it is essential that elutriation be accomplished in a way which retains the desired amount of combustible deposition on the pellets; consequently, the elutriating gas must be a noncombusting gas, that is, a gas that will not burn the combustible deposition on the pellets.

The mixtures of pellets and spent shale in line 42 passes to elutriation zone 46 where the mixture is subjected to gas elutriation with gas entering the elutriation zone by way of line 45. As stated previously, at least a part of this elutriation gas is hot flue gas from the pellet deposition burning zone. In the elutriation zone, a major portion of the spent shale is elutriated and separated from the pellets. For simplicity, only one elutriation stage is shown and the spent shale and elutriation gas pass overhead through line 47 to overhead separator 48, for example, a cyclone, scrubber, or the like, to remove the spent shale to disposal by way of bottom line 49 and to clean and allow the elutriation gas to exit overhead through line 15 and pass from the separation zone through line 15 to pellet lift system 12 as previously described. If additional elutriation stages are used, the elutriation gas may first be returned to such stages, if desired. The mixture of remaining pellets and spent shale in elutriation zone 46 is added to any other pellet return streams (not shown) and passed through line 11 to pellet lift system 12, where, as previously described, the pellets are relifted to-the pellet deposition burning zone by the hot lift gas from line 15 at least part of which is comprised of hot flue gas originating in the pellet deposition burning zone.

In a retorting system of the nature just described, it would be highly advantageous to initially produce the hot flue gas at a pressure which would eliminate or at least reduce the need for subsequent pressure increasing or compession stages especially at points where the gas contains entrained solids. This may be accomplished in the illustrated process by raising the inlet pressure of the combustion supporting gas in line 26 when the combustion supporting is more or less solids free. It has previously been pointed out that the pressure in the retort zone may be the autogenous pressure. This retort zone pressure may be below the pressure .to which the combustion supporting gas is compressed. If so, an appropriate conventional gas seal system will be placed in the pellet feed system between the pellet deposition burning zone and the retort zone. Preferably, the combustion supporting gas will be compressed to a pressure high enough to cause passage of the flue gas throughout the system as herein described. For example, if the retort system involves one lift heating unit in the secondary preretort heating zone, three stages of elutriation, and two lift heating units in the first preretort heating zone, the entry pressure of the combustion supporting gas could be increased to about 6 to 19 pounds per square inch above the atmospheric pressure.

The foregoing description of the conditions and variables of the process illustrates a preferred method of conducting retort system and how the system conserves gas heat energy, and reduces emission points to accomplish the advantages and objectives herein set forth.

Reasonable variations and modifications are practical with the scope of this disclosure without departing from the spirit and scope of the claims of this invention.

For example, while the disclosure of this process and the variables have been limited to oil shale, the process concepts lend themselves readily to retorting any solid organic carbonaceous material containing hydrocarbon values which can be recovered by thermal vaporization of the solid carbonaceous material, such as, for example, coal, peat, and tar sands. By way of further example, while only a single train of units and stages have been described, it is to be understood that any stage or zone could be comprised of more than one stage or zone.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

l. A method for retorting of crushed oil shale comprising a. lifting pellets bearing a combustible deposition with lift gas to a pellet deposition burning zone, said pellets being particulate heat carriers in a size range between approximately 0.06 inch and 0.3 inch;

b. heating said pellets passed to said pellet deposition burning zone to an outlet temperature of between 1,000 F and l,500 F by burning said combustible deposition on said pellets with a combustion supporting gas thereby producing hot flue gas;

c. passing said lift gas from said pellet deposition burning zone to an oil shale first preretort heating zone;

d. feeding crushed oil shale at a temperature lower than said lift gas to said first preretort heating zone and heating said oil shale by intimately contacting said oil shale with said lift gas to a temperature below about 600 F;

e. passing the heated oil shale from said first preretort heating zone to a second preretort heating zone;

f. passing said hot flue gas from said pellets deposition burning zone to said second preretort heating zone and heating at a temperature higher than the heated oil shale passed thereto and heating said oil shale by intimately contacting said heated oil shale with said hot flue gas to a temperature below about 600 F;

g. passing said heated pellets from said pellet deposition burning zone and the heated oil shale from said second preretort heating zone to a retort zone, said pellets having a surface area of at least 10 square meters per gram, the ratio of said heated pellets to said crushed oil shale entering said retort zone on a weight basis being between 1 and 3, said ratio also being such that the sensible heat in said pellets is sufficient to provide at least 50 percent of the heat required to heat said oil shale from its retort zone feed temperature to a retort zone outlet temperature of between 800 F and l,l50 F;

h. retorting in said retort zone gas and oil products from the oil shale thereby forming particulate spent shale;

i. recovering said gas and oil products generated by retorting the oil shale;

j. passing hot flue gas from said second preretort heating zone to a separation zone;

k. passing a mixture of pellets and spent shale from said retort zone to said separation zone at least one stage of which involves subjecting said mixture to elutriate and separate spent shale from the pellets in said mixture with elutriation gas, said elutriation gas being comprised of said hot flue gas passed to said separation zone;

1. passing separated pellets from said separation zone to a pellet lift zone;

m. passing hot elutriation gas from said separation zone to said pellet lift zone, said hot elutriation gas being comprised of said hot flue gas passed from said pellet deposition burning zone to said second preretort heating zone to said separation zone; and

n. using said hot elutriation gas passed to said pellet lift zone as lift gas in step (a). 2. The method according to claim 1 wherein prior to step (j), the hot flue gas from said second preretort through a carbon monoxide shift reaction zone. 

2. The method according to claim 1 wherein prior to step (j), the hot flue gas from said second preretort heating zone is passed through a supplemental heating zone wherein said hot flue gas is heated to a higher temperature.
 3. The method according to claim 1 wherein the hot flue gas contains carbon monoxide, and at some point in the method between the pellet deposition burning zone and the end of step (d), said hot flue gas is passed through a carbon monoxide shift reaction zone. 